1. Field of the Invention
This invention pertains to submerged multi-purpose facilities, such as a manned submersible drilling and production structure used in offshore oil and gas production.
2. Description of the Prior Art
An examination of marine structures useful in providing a working facility for industrial application reveals that such structures have been designed in the past to withstand or protect against the natural marine forces acting upon the structure. For example, offshore drilling platforms designed to provide a working facility above the surface of the water must be designed to overcome the tremendous lateral forces of wave action and currents that constantly lash at the superstructure. In addition, the support legs have to be sufficiently ruggedly built so that they can withstand the enormous hydrostatic pressures that are present all of the way to the bottom of the sea.
Improvements on such a basic theme have included breakwaters to reduce the lateral forces before they hit against the superstructure and semi-submersible structures which cause at least some of the structures to escape some of the heaviest forces on the surface.
Drilling ships have also been used. Such ships have to be anchored or constantly dynamically corrected to maintain the ship in a relatively constant position. The anchoring or mooring lines are generally exposed to the same large lateral forces discussed above. In addition, the use of such lines have to include a means for adjusting the lengths as the ship rises and falls. Dynamic repositioning using orthogonally driven propellers means that such a system is always exposed to the possibility that a motor will stop. Moreover, a ship which is buoyant on or just below the surface must include means for vertically adjusting the length of the risers and other appendages attached to the structure and to the sea bottom.
Many attempts are shown in the prior art for at least partially avoiding the surface and near-surface lateral forces by using a submersible or partly submersible structure. For example, U.S. Pat. No. 3,455,115, Burrus, reveals a submersible chamber which is tethered to the bottom via a float-pulley combination that permits its precise location or positioning. The structure is operably positionable through such a technique between a location near the bottom to above the surface. Normally, the location of the chamber is near the bottom. The float-pulley combination provides means for raising the chamber for maintenance purposes.
U.S. Pat. No. 3,638,720, Thomas, discloses a submerged capsule for servicing many wells at a intermediate production zone. The use of cables for tie-down purposes is recognized, but the location of the structure is not optimized. U.S. Pat. No. 3,656,307, Mott, reveals a sub-sea installation connected to the bottom of the sea. A buoyant mast extends upwards therefrom to assume a desirable location, which is below the surface, with rigid conduits projecting above the water surface. No optimal location is suggested for the buoyant mast.
U.S. Pat. No. 3,766,874, Helm, et al., shows a two-section barge, one section being below the water line and one section being above. The joinder of the two sections is by a line below the water surface. The structure shown in U.S. Pat. No. 3,771,484, Schott, et al., is an inflatable island suitable for providing a landing strip. There is a submerged portion of the structure; however, it is not optimized in location other than to keep the island suitably above the surface of the water.
The structure shown in U.S. Pat. No. 3,902,533, Jergins, is a submersible rig. The rig is secured to the bottom, not at an intermediate level between the bottom and the surface. Jergins did recognize that at lower depths, the rig must withstand enormous inward pressures, but no solution is given.
U.S. Pat. No. 4,473,323, Gregory, discloses a buoyant substructure underneath the surface of the water, with a drilling platform above the sea surface. The location of the overall structure is in the area for carrying the brunt of lateral surface wave action. The structure shown in U.S. Pat. No. 4,004,531, Mott, is similar to the elements of the structure shown in Gregory patent '323. In addition, a protective caisson is inclined to protect the riser pipe from the submerged structure to the drilling deck. Nothing is optimized with respect to vertical location.
U.S. Pat. No. 4,212,561, Wipkink, shows two vertically spaced separate structures, at least one of which is submerged. The structures are connected together with relative movement in mind. The connections include ducts for fluid and the like, but the location of the submerged structure is not optimized.
In contrast to the approach taken in the prior art of withstanding the environment, the Submerged Multi-Purpose Facility disclosed herein is a structure which provides a working marine environment which takes advantage of the natural forces in its design and installation.
Therefore, it is a feature of the present invention to provide a submerged multi-purpose facility which is located at an improved optimal depth location regarding the natural lateral forces existing in the marine location where the facility is positioned for operations.
It is another feature of the present invention to provide an improved submerged multi-purpose facility which is located optimally with regard both lateral forces and hydrostatic pressure forces.
It is still another feature of the present invention to provide an improved submerged multi-purpose facility which does not require an extremely rugged support structure, but satisfactorily employs anchored tethers and a balanced buoyant/ballast to keep the facility in location.